Messages in this thread | | | Date | Wed, 17 Apr 2024 22:19:31 -0700 | Subject | Re: [PATCH v1 18/31] x86/resctrl: Allow resctrl_arch_mon_event_config_write() to return an error | From | Reinette Chatre <> |
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Hi Dave,
On 4/17/2024 7:42 AM, Dave Martin wrote: > Hi Rainette, > > On Thu, Apr 11, 2024 at 10:39:37AM -0700, Reinette Chatre wrote: >> Hi Dave, >> >> On 4/11/2024 7:17 AM, Dave Martin wrote: >>> On Mon, Apr 08, 2024 at 08:23:36PM -0700, Reinette Chatre wrote: >>>> Hi James, >>>> >>>> On 3/21/2024 9:50 AM, James Morse wrote: >>>>> resctrl_arch_mon_event_config_write() writes a bitmap of events provided >>>>> by user-space into the configuration register for the monitors. >>>>> >>>>> This assumes that all architectures support all the features each bit >>>>> corresponds to. >>>>> >>>>> MPAM can filter monitors based on read, write, or both, but there are >>>>> many more options in the existing bitmap. To allow this interface to >>>>> work for machines with MPAM, allow the architecture helper to return >>>>> an error if an incompatible bitmap is set. >>>>> >>>>> When valid values are provided, there is no change in behaviour. If >>>>> an invalid value is provided, currently it is silently ignored, but >>>>> last_cmd_status is updated. After this change, the parser will stop >>>>> at the first invalid value and return an error to user-space. This >>>>> matches the way changes to the schemata file are made. >>>>> >>>> >>>> Is this needed? With move of mbm_cfg_mask to rdt_resource I expect >>>> MPAM would use it to set what the valid values are. With that done, >>>> when user space provides a value, mon_config_write() compares user >>>> provided value against mbm_cfg_mask and will already return early >>>> (before attempting to write to hardware) with error >>>> if value is not supported. This seems to accomplish the goal of this >>>> patch? >>> >>> This sounds plausible. >>> >>> In a recent snapshot of James' MPAM code, it looks like we could be >>> initialising rdt_resource::mbm_cfg_mask when setting up the rdt_resource >>> struct for resctrl, though in fact this information is captured >>> differently right now. I'm sure why (though James may have a >>> reason). [1] >>> >>> I don't see an obvious reason though why we couldn't set mbm_cfg_mask >>> and detect bad config values globally in mon_config_write(), the same as >>> for the existing AMD BMEC case. >>> >>> Nothing in the MPAM architecture stops hardware vendors from randomly >>> implementing different capabilities in different components of the >>> system, but provided that we only expose the globally supported subset >>> of event filtering capabilities to resctrl this approach looks workable. >>> This consistent with the James' MPAM code deals with other feature >>> mismatches across the system today. >>> >>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_resctrl.c?h=mpam/snapshot/v6.7-rc2#n730 >> >> My response was based on what I understood from the goal of this change >> as described by the changelog. The patch does not appear to match with >> the goals stated in changelog. >> >> As I understand the patch it aims to detect when there is an invalid >> event id. It is not possible for this scenario to occur because this code >> is always called with a valid event id. >> >> Reinette > > I guess this will need discussion with James. FWIW, my impression was > that the real goal of this patch was to allow a bad event config to be > detected at cross-call time and reported asynchronously. Changes > elsewhere look to be there just to make error reporting consistent for > other existing paths too.
How do you interpret "bad event config"?
As I understand it, this patch only sets an error in one scenario:
index = mon_event_config_index_get(mon_info->evtid); if (index == INVALID_CONFIG_INDEX) { pr_warn_once("Invalid event id %d\n", mon_info->evtid); mon_info->err = -EINVAL; return; }
When will mon_info->evtid be anything but QOS_L3_MBM_TOTAL_EVENT_ID or QOS_L3_MBM_LOCAL_EVENT_ID?
Reinette
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